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Community and Q&A

Underground Roof and Gutter

manbeardo | Posted in General Questions on

One of the side yards on our house is 4′ wide and bordered by the retaining wall dividing our property from our uphill neighbor’s. When we bought the house, the side yard was covered in pavers. We’d like to install a lawn in that yard for our dog.

I recently removed the pavers and tested watering the side yard with a soaker hose. The results were pretty terrible. The soil drained very slowly and built up a large pool. Then, a lot of that water made its way into the basement through a small gap between the slab and a bench footing that appears to have been added at some point after original construction.

Since the main way moisture got into the basement was from under the slab, my best guess here is that the drain tile failed. Replacing the drain tile seems prohibitively expensive because we’d need to dig a 6′ deep trench in a 4′ wide space. I’ve read a few articles about underground roofs and ground gutters, which seem like they could be a good solution here, but the close proximity of the neighbor’s retaining wall has me concerned. Does the narrow yard create any unique considerations for an underground roof that I’m missing?

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  1. plumb_bob | | #1

    You can seal that area with a "roof" but ultimately the water need somewhere to go. If you do seal off the ground you have those basement windows to worry about taking in water.

    Ultimately, improving the drainage in that area will be best for your house, yard and probably dog.

  2. GBA Editor
    Kiley Jacques | | #2

    Peter Yost of Building Wright (and formerly BuildingGreen) has done what it sounds like you are describing, and in this article talks about flashing the house to the ground (look for the detail in the photo gallery): An Underground Roof?.

    1. manbeardo | | #6

      That article and Michael Maines' article about ground gutters are the reference materials I've been looking at. I'm curious if any of the design principles they employed are complicated by the narrow yard. Since the retaining wall is uphill, I suppose there's still plenty of soil underground in the neighbor's lot to take on some of the water. If it was a downhill retaining wall, it seems like an underground roof could be very effective at liquefying the soil around its perimeter.

  3. PLIERS | | #3

    Is that there so the dog can go to the bathroom? What other yard space do you have? I have a similar tight alleyway between my neighbor. We have a long concave concrete swale. I think that’s what you call it. Water gets flushed away from foundation. You would need something serious to keep water out of there, it’s a low spot and water has no where to go.

    1. manbeardo | | #7

      Our other yard space is a backyard that's completely covered by a concrete pad and a slightly larger (and lower) side yard that we're using for construction debris because it has a door to the basement. I'm trying to set up something for the dog to use as a potty station for the next few years before we get to the bigger landscaping portions of our renovation project.

  4. Jon_R | | #4

    +1 on a concave underground roof and slope such that water will flow to the front or the back (and not downward). If this is impossible, then consider a sump and pump.

    Hard to imagine grass growing well there - which might mean mostly mud. Perhaps the dog would accept gravel? Or fake grass?

  5. manbeardo | | #5

    The side yard is relatively dry (for Seattle) most of the year because it's ~50% covered by our roof overhang and gutters. It's fairly easy to drain to daylight because we have a significant slope on both the minor and major axis of the lot. The high side of the sidewalk is ~3ft lower than this side yard.

    In the longer term, I might consider diverting some of the water from the gutters to the lawn since we're in a combined sewer neighborhood (stormwater goes directly to the sewer). Replacing our impermeable surfaces with vegetation is important not just to our dog, but to the city as well.

  6. dan_saa | | #8

    If you can daylight drainage, how about removing 18" of soil temporarily, installing a pond liner with layer of washed gravel on top that ties into drainage & outlet, cover with geotextile/filter fabric, then soil. Shallow rooted vegetation should grow and excess water will get diverted to street.

    1. Expert Member
      Michael Maines | | #9

      That's exactly what I would recommend. I'd rather see clay or a clay-loam mix topped with 3" stone at grade, but to meet the doggy-restroom requirement, your approach should work. Slope the pond liner (EPDM would be my choice, 0.045" or preferably 0.060") toward the retaining wall with a perforated pipe at the wall.

      1. manbeardo | | #11

        When you say to put the perforated pipe "at the [retaining] wall", do you mean directly next to it or on the retaining wall's side of the EPDM? Wouldn't it be at risk of collapse if I dug 18" down directly at its base?

        1. dan_saa | | #12

          The perf drain pipe is to collect the water along the length and more quickly drain it away to daylight. You could put on the side or in the center of the yard sloping the ground towards the drain pipe. The pipe will need washed gravel around it with filter fabric. Research french drains. Also see NDS product below.

          I think the 18" depth of soil could be reduced to say 8" soil plus 2" gravel, but you should do research based on your planting. (my link below turned up 8" for 1st page). For a 3" pipe plus 2" gravel around, this puts the bottom of excavation a minimum of 8"+7"=15".

          The excavation should not go below the bottom of any footing, and preferable not below top of footings to be safe for lateral movement. Without knowing all the details of grading, bottom of footing, etc., it is hard to get too specific.

    2. Expert Member
      MALCOLM TAYLOR | | #10


      Good plan. I bet it would transform that small side-yard.

  7. Dinnyrishell | | #13

    Rain streams coming off the roof and snowmelt masses become a real disaster for roofs without drainage systems. The covering of the yard is being destroyed, and plantings are dying due to waterlogging. The close location of the groundwater reservoir or the seasonal passage of pressure streams can render the foundation of a house unusable for years. Installation of drainage systems and competent drainage organization roofs from the site neutralizes the inconveniences caused by moisture and maintains an optimal balance of humidity in the adjacent territory.

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